Foreign Secretary Philip Hammond says the killing of James Foley is an "utter betrayal of our country, our values and everything the British people stand for" Getty

Philip Hammond has claimed intelligence agencies are “getting warm” in the investigation to reveal the identity of the Islamic State (Isis) militant known as Jihadi John.

The man, who speaks with a London or south-east accent, is believed to be the masked murderer of David Haines, Steven Sotloff and James Foley seen in the extremist group’s gruesome beheading videos.

Officials have claimed Britain is “close” to identifying the terrorist for a month but no identity has been released.

Speaking to CNN at the United Nations summit in New York, the Foreign Secretary claimed the net was closing in.

“We are getting warm, we are working on all the leads. There's a big investigation and we are getting warm,” he said.

"We are narrowing down the field but I don't want to say any more at this stage.“

Up to 500 British jihadists are believed to be fighting with Isis in Syria and Iraq and the Government has increased counter-extremism measures in an attempt to prevent anyone else travelling to the conflict zones.

One of the men believed to be under investigation is a former rapper who reportedly travelled to the Syria last year from west London to join Isis.

Abdel-Majed Abdel Bary, 24, known as L Jinny or Lyricist Jinn, came to national attention earlier this year, when he posted a picture of himself holding a severed head on Twitter.

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Numerous reports have followed in several outlets that the identities of Jihadi John and his associates are known to authorities but they cannot act without endangering the lives of Alan Henning and other hostages.

The Foreign Secretary condemned the treatment of the volunteer aid worker, who left his family in Salford to help Syrian refugees.

In pictures: The rise of Isis

In pictures: The rise of Isis

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Isis fighters

Fighters of the Islamic State wave the group's flag from a damaged display of a government fighter jet following the battle for the Tabqa air base, in Raqqa, Syria

AP

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Isis fighters

Fighters from Islamic State group sit on their tank during a parade in Raqqa, Syria

AP

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Isis fighters

Fighters from the Islamic State group pray at the Tabqa air base after capturing it from the Syrian government in Raqqa, Syria

AP

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Isis fighters

Fighters from extremist Islamic State group parade in Raqqa, Syria

AP

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Isis kidnapping

A video uploaded to social networks shows men in underwear being marched barefoot along a desert road before being allegedly executed by Isis

Getty Images

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Isis kidnapping

Haruna Yukawa after his capture by Isis

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Isis kidnapping

Khalinda Sharaf Ajour, a Yazidi, says two of her daughters were captured by Isis militants

Washington Post

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Isis fighters

Spokesperson for Isis

Vice News via Youtube

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A pro-Isis leaflet

A pro-Isis leaflet handed out on Oxford Street In London

Ghaffar Hussain

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Isis fighters

Isis Jihadists burn their passports

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Isis controls Syrian Aid

A man collecting aid administered by Isis in Syria

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Isis controls Syrian Aid

A woman collecting aid administered by Isis in Syria

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Isis controls Syrian Aid

Local civilians queue for aid administered by Isis. Since it declared a caliphate the group has increasingly been delivering services such as healthcare, and distributing aid and free fuel

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Iraq crisis

Iraqi security forces detain men suspected of being militants of the Isis group in Diyala province

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Iraq crisis

Mourners carry the coffin of a Shi'ite volunteer from the brigades of peace, who joined the Iraqi army and was killed during clashes with militants of the Isis group in Samarra, during his funeral in Najaf

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Iraqi refugees

An Iraqi Shiite Turkmen family fleeing the violence in the Iraqi city of Tal Afar, west of Mosul, arrives at a refugee camp on the outskirts of Arbil, in Iraq's Kurdistan region

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Isis leader Abu Bakr al-Baghdadi

A photograph made from a video by the jihadist affiliated group Furqan Media via their twitter account allegedly showing Isis leader Abu Bakr al-Baghdadi delivering a sermon during Friday prayers at a mosque in Mosul. Abu Bakr al-Baghdadi declared an Islamist caliphate in the territory under the group's control in Iraq and Syria

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Islamic extremists destroying mosques in Iraq

Shiite's Al-Qubba Husseiniya mosque explodes in Mosul

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Islamic extremists destroying mosques in Iraq

Smoke and debris go up in the air as Shiite's Al-Qubba Husseiniya mosque explodes in Mosul. Images posted online show that Islamic extremists have destroyed at least 10 ancient shrines and Shiite mosques in territory - the city of Mosul and the town of Tal Afar - they have seized in northern Iraq in recent weeks

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Islamic extremists destroying mosques in Iraq

A bulldozer destroys Sunni's Ahmed al-Rifai shrine and tomb in Mahlabiya district outside of Tal Afar

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Iraq crisis

Iraqi security forces celebrate after clashes with followers of Shiite cleric Mahmoud al-Sarkhi, in front of his home in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad

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Iraq crisis

Iraqi security forces arrest a follower of Shiite cleric Mahmoud al-Sarkhi after clashes with his followers in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad

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Iraq crisis

Iraqi security forces arrest a follower of Shiite cleric Mahmoud al-Sarkhi at his home after clashes with his followers in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad

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Iraq crisis

Iraqi security forces arrest a follower of Shiite cleric Mahmoud al-Sarkhi after clashes with his followers in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad

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Iraq crisis

A vehicle burns in front of a home of a follower of Shiite cleric Mahmoud al-Sarkhi after clashes with his followers in the Shiite holy city of Karbala, 50 miles (80 kilometers) south of Baghdad

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Iraqi refugees

An Iraqi woman holds her exhausted son as over 1000 Iraqis who have fled fighting in and around the city of Mosul and Tal Afar wait at a Kurdish checkpoint in the hopes of entering a temporary displacement camp in Khazair

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Iraqi refugees

Displaced Iraqi women hold pots as they queue to receive food during the first day of the Islamic holy month of Ramadan, at an encampment for displaced Iraqis who fled from Mosul and other towns, in the Khazer area outside Irbil, north Iraq

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Isis fighters in Syria

A militant Islamist fighter waving a flag, cheers as he takes part in a military parade along the streets of Syria's northern Raqqa. The fighters held the parade to celebrate their declaration of an Islamic "caliphate" after the group captured territory in neighbouring Iraq

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Isis fighters in Syria

Isis fighters wave flags as they take part in a military parade along the streets of Syria's northern Raqqa province

Reuters

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Isis fighters in Syria

Isis fighters travel in a vehicle as they take part in a military parade along the streets of Syria's northern Raqqa province

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Isis fighters in Syria

Fighters from the Isis group during a parade with a missile in Raqqa, Syria. Militants from an al-Qaida splinter group held a military parade in their stronghold in northeastern Syria, displaying U.S.-made Humvees, heavy machine guns, and missiles captured from the Iraqi army for the first time since taking over large parts of the Iraq-Syria border

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Isis fighters in Syria

Isis fighters during a parade in Raqqa, Syria

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Isis fighters in Syria

Fighters from the Isis group during a parade in Raqqa, Syria. Militants from the splinter group held a military parade in their stronghold in northeastern Syria, displaying U.S.-made Humvees, heavy machine guns, and missiles captured from the Iraqi army for the first time since taking over large parts of the Iraq-Syria border

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Isis fighters in Syria

Isis fighters hold a military parade in their stronghold in northeastern Syria

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Isis fighters in Syria

Isis fighters during a parade in Raqqa, Syria

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Isis fighters in Syria

A member loyal to the Isis waves an Isis flag in Raqqa

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Iraq crisis

Iraqi anti-government gunmen from Sunni tribes in the western Anbar province march during a protest in Ramadi, west of Baghdad. The United Nations warned that Iraq is at a "crossroads" and appealed for restraint, as a bloody four-day wave of violence killed 195 people. The violence is the deadliest so far linked to demonstrations that broke out in Sunni areas of the Shiite-majority country more than four months ago, raising fears of a return to all-out sectarian conflict

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Iraq crisis

Iraqi security forces hold up a flag of the Isis group they captured during an operation to regain control of Dallah Abbas north of Baqouba, the capital of Iraq's Diyala province, 35 miles (60 kilometers) northeast of Baghdad

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Isis fighters in Iraq

Isis fighters parade in the northern city of Mosul

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Iraq crisis

Volunteers, who have joined the Iraqi army to fight against the predominantly Sunni militants from the radical Isis group, demonstrate their skills during a graduation ceremony after completing their field training in Najaf

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Iraq crisis

Kurdish Peshmerga troops fire a cannon during clashes with militants of the Isis group in Jalawla, Diyala province

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Lieutenant General Qassem Atta speaks during a press conference

Iraqi Prime Minister's security spokesman, Lieutenant General Qassem Atta speaks during a press conference about the latest military development in Iraq, in the capital Baghdad. Iraqi forces pressed a campaign to retake militant-held Tikrit, clashing with jihadist-led Sunni militants nearby and pounding positions inside the city with air strikes in their biggest counter-offensive so far

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A police station building destroyed by Isis fighters

An exterior view of a police station building destroyed by gunmen in Mosul city, northern Iraq. Iraq's new parliament is expected to convene to start the process of setting up a new government, despite deepening political rifts and an ongoing Islamist-led insurgency. Iraqi President Jalal Talabani issued a decree inviting the new House of Representatives to meet and form a new government

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Isis fighters in Iraq

Smoke billows from an area controlled by the Isis between the Iraqi towns of Naojul and Tuz Khurmatu, both located north of the capital Baghdad, as Iraqi Kurdish Peshmerga forces take part in an operation to repel the Sunni militants

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Iraqi refugees

An elderly Iraqi woman is helped into a temporary displacement camp for Iraqis caught-up in the fighting in and around the city of Mosul in Khazair

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Iraqi refugees

An Iraqi Christian woman fleeing the violence in the village of Qaraqush, about 30 kms east of the northern province of Nineveh, cries upon her arrival at a community center in the Kurdish city of Arbil in Iraq's autonomous Kurdistan region

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Iraqi refugees

An Iraqi woman, who fled with her family from the northern city of Mosul, prays with a copy of the Quran

AP

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Isis fighters in Iraq

The body of an Isis militant killed during clashes with Iraqi security forces on the outskirts of the city of Samarra

Reuters

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Iraq crisis

Iraqi civilians inspect the damage at a market after an air strike by the Iraqi army in central Mosul

EPA

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Iraq crisis

Members of the Al-Abbas brigades, who volunteered to protect the Shiite Muslim holy sites in Karbala against Sunni militants fighting the Baghdad government, parade in the streets of the city

AP

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Iraq crisis

Shia tribesmen gather in Baghdad to take up arms against Sunni insurgents marching on the capital. Thousands have volunteered to bolster defences

AFP/Getty

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Iraq crisis

A van carrying volunteers joining Iraqi security forces against Jihadist militants. Prime Minister Nuri al-Maliki announced the Iraqi government would arm and equip civilians who volunteered to fight

AFP/Getty

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Iraq

Fighters of the Isis group parade in a commandeered Iraqi security forces armored vehicle down a main road at the northern city of Mosul

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Iraq

An Islamist fighter, identified as Abu Muthanna al-Yemeni from Britain (R), speaks in this still image taken undated video shot at an unknown location and uploaded to a social media website. Five Islamist fighters identified as Australian and British nationals have called on Muslims to join the wars in Syria and Iraq, in the new video released by the Isis

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Iraq

Al-Qa’ida inspired militants stand with captured Iraqi Army Humvee at a checkpoint belonging to Iraqi Army outside Beiji refinery some 250 kilometers (155 miles) north of Baghdad. The fighting at Beiji comes as Iraq has asked the U.S. for airstrikes targeting the militants from the Isis group. While U.S. President Barack Obama has not fully ruled out the possibility of launching airstrikes, such action is not imminent in part because intelligence agencies have been unable to identify clear targets on the ground, officials said

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Iraq

Militants attacked Iraq's main oil refinein Baiji as they pressed an offensive that has seen them capture swathes of territory, a manager and a refinery employee said

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Iraq

Militants from the Isis group parading with their weapons in the northern city of Baiji in the in Salaheddin province

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Iraq

A smoke rises after an attack by Isis militants on the country's largest oil refinery in Beiji, some 250 kilometers (155 miles) north of the capital, Baghdad. Iraqi security forces battled insurgents targeting the country's main oil refinery and said they regained partial control of a city near the Syrian border, trying to blunt an offensive by Sunni militants who diplomats fear may have also seized some 100 foreign workers

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Iraq

Militants of the Isis group stand next to captured vehicles left behind by Iraqi security forces at an unknown location in the Salaheddin province. For militant groups, the fight over public perception can be even more important than actual combat, turning military losses into propaganda victories and battlefield successes into powerful tools to build support for the cause

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Iraq

An injured fighter (C) from the Isis group after a battle with Iraqi soldiers at an undisclosed location near the border between Syria and Iraq

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Iraq

Fighters from the Isis aiming at advancing Iraqi troops at an undisclosed location near the border between Syria and Iraq

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Iraq

Fighters from the Isis group taking position at an undisclosed location near the border between Syria and Iraq

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Iraq

Fighters from the Isis group inspecting vehicles of the Iraqi army after they were seized at an undisclosed location near the border between Syria and Iraq

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Iraq

One Iraqi captive, a corporal, is reluctant to say the slogan, and has to be shouted at repeatedly before he obeys

Sky News

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Iraq

Iraqi captives held by the extremists

Sky News

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Iraq

Iraqi captives held by the extremists

Sky News

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Iraq

Militants of the Isis group force captured Iraqi security forces members to the transport

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Iraq

Militants of the Isis group transporting dozens of captured Iraqi security forces members to an unknown location in the Salaheddin province ahead of executing them

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Iraq

A major offensive spearheaded by Isis but also involving supporters of executed dictator Saddam Hussein has overrun all of one province and chunks of three others

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Iraq

Militants of the Isis group executing dozens of captured Iraqi security forces members at an unknown location in the Salaheddin province

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Iraq

Isis militants taking position at a Iraqi border post on the Syrian-Iraqi border between the Iraqi Nineveh province and the Syrian town of Al-Hasakah

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Iraq

Isis rebels show their flag after seizing an army post

AFP/Getty Images

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Iraq

Isis militants waving an Islamist flag after the seizure of an Iraqi army checkpoint in Salahuddin

Getty Images

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Iraq

Demonstrators chant slogans as they carry al-Qa’ida flags in front of the provincial government headquarters in Mosul, 225 miles (360 kilometers) northwest of Baghdad. In the week since it captured Iraq's second-largest city, Mosul, a Muslim extremist group has tried to win over residents and has stopped short of widely enforcing its strict brand of Islamic law, residents say. Churches remain unharmed and street cleaners are back at work

In a cruel twist, the Isis militants holding Mr Henning sent his wife an audio recording of him pleading for his life.

Mr Hammond said: "What we do know is Alan Henning was out there simply to help. He was a humanitarian worker who went with an aid convoy organised by British Muslim groups to provide support to refugees in Syria.

“He was doing good and I think it speaks volumes about Isis that this is the way they are treating somebody who went out there to help their people and support their communities in their time of need.”

Mr Hammond’s claims come as the United States and Arab allies continue bombing Isis strongholds in Syria, reportedly including the city of Raqqa where it is thought Mr Henning is being held.

Separate footage released this week showed British photojournalist John Cantlie in a propaganda video reading a speech scripted by Isis.

When asked about Britain’s policy of not paying ransoms to terrorists to release hostages, Mr Hammond said it was “absolutely right”.

The Home Office would not comment further on the investigation and it is understood that, for security reasons, Jihadi John’s identity would not be made public if it was known.